Hello matplotlib-community,

I just realized a visual glitch problem in the corners of the axes borders. If the linewidth is large, white corners are visible. That code-snippet should illustrate the problem:

import matplotlib.pylab as plt
plt.rcParams.update({"axes.linewidth": 20.0 })
plt.plot(range(5))
plt.show()

I guess the problem is due to the capstyle property of the axes borders, which seems to be "butt". If the capstyle is "projecting" or "round", it will solve the problem. Maybe one can introduce a rc param option to choose the capstyle.